Experimental Electricity 



Practical Hints 

 for the Amateur 



Wireless 

 Communication 



A Microphonic Amplifier 



By Arthur Ellison 



THE amplification of radio signals is 

 receiving much attention. For 

 this purpose nothing is superior 

 to the audion amplifier, but the price of 

 this apparatus places it beyond the 

 reach of many amateurs. 



The microphone amplifier herein de- 

 scribed will do remarkable work when 

 properh- adjusted, but it 7)utsl be kept 

 free from vibration. The easiest way 

 to do this, and perhaps the best, is to 

 place several layers of felt under the 

 base, or, if possible, to mount it firmh- on 

 a solid brick wall. Small, soft rubber 

 feet under the in- 

 strument are also of 

 some assistance in 

 cutting out undue 

 vibration. 



In the accom- 

 panying drawing 

 at Fig. I is shown the 

 electromagnetic 

 microphone com- 

 plete. It consists of 

 two I ,ooo-ohm tele- 

 phone receivers 

 mounted on a base 

 in a vertical po- 

 sition. 



The recei\'er 

 shells aredrilled and 

 tapped toadmit two 

 small machine 

 screws which fasten 

 them to the brass 

 angles. The support 



Fig. 1. The Electromagnetic Microphone 

 Complete and Mounted in Position 



437 



on the left is a plain L with a hole in the 

 bottom part to clamp it to the base, but 

 the other L is somewhat different in 

 shape and is shown in detail in Fig. 40. As 

 will be readily understood, it is slightly 

 shorter than the first named, and is 

 turned up at the end so the thumb screw 

 will act on it. A slot is cut in the bottom 

 part so that the recei\'er may be moved 

 forward or backward by operating the 

 thumbscrew, and a spring is arranged to 

 push it back normally. 



In the center of each of the dia- 

 phragms is mounted a small carbon 

 button. These are 

 only about J^ in. in 

 diameter and are 

 shown in detail in 

 Fig. 2. A conical 

 hole is drilled in one 

 face and a slot cut 

 with a fine saw on 

 one side. A very 

 thin flexible wire is 

 laid in this slot and 

 solder poured in to 

 hold it there. By 

 twisting the saw 

 while cutting the 

 slot, the slot will 

 be made wider at 

 the bottom, and the 

 solder will hold the 

 wire in place. 



The carbon but- 

 tons are fastened 

 to the center of the 



